1. Field of the Invention
The present invention relates to the transmission of electrical carrier signals, and to the recording and reproduction of sound, and in particular to the reproduction, and the rerecording and playback, of previously recorded sound signals.
2. Description of the Prior Art
In playing back a sound recording, one hears not only a reproduction of the original signal, but also the undesirable extraneous noise components that are caused by imperfections in the original recording and in the storage medium, damage to the storage medium, and inherent defects inadvertently introduced by the metal parts used in the manufacture of the storage medium. Such noises are commonly designated by such terms as "surface noise", "crackle", "ticks", etc. These noises are generally of a first type of an audibly continuous nature distributed fairly uniformly throughout the audio frequency range and known as "white" noise, and of a second type which are transient sounds of short duration and high amplitude relative to the signal.
Radio broadcast signals also may contain some additional noise components caused by disturbances in the transmission or reception, commonly called "static". Static, also, typically consists of pulses of short duration and high amplitude.
The development of two-channel stereophonic record playing equipment makes it possible to play back monophonic disc or cylinder recordings, of either the vertical-cut or the lateralcut type, in such a manner that one channel or the reproduction system reproduces, essentially, from one wall of the groove, and the other channel of the reproduction reproduces, essentially, from the other wall of the groove, both walls of the groove bearing the identical recorded carrier signal.
However, such factors as imperfections in the record material, defects in manufacture, damage, dirt, wear, mildew, etc. which may introduce transient noises into the reproduction of the recording, will not be identical on opposite sides of the record groove. Therefore, in reproducing a monophonic recording with stereophonic playback equipment, the carrier signal will be identical in both channels, save for such effects introduced in playback such as tracing error, tracking error and "pinch effect" that turn out in the application of this invention, to be of a second order nature, whereas noise transients will be different in the two channels.
Previous processes in reducing unwanted noises in sound reproduction have relied on restricting the frequency range as far as is feasible for the purpose or application. Typically, the higher frequency components of the noise, usually referred to by such terms as "surface noises", "crackle", "ticks", etc. are suppressed throughout the playback, usually at some sacrifice to the fidelity of reproduction of the recorded signal. Some sophistication in this process was introduced by Harmon H. Scott whose Dynamic Noise Suppressor used electronic means automatically to alter the frequency pass band width in record reproduction in accordance with the loudness of the music. The Scott Suppressor takes advantage of the phenomena that the relative sensitivity of the ear to various frequencies varies with the loudness of the sound, and that louder passages of music and other audio signals have a correspondingly improved masking effect on the surface noise perceived from the recording; so that, in louder passages a wider frequency range is feasible in reproduction than is the case in the reproduction of the quieter passages for similar listening comfort and perceived tone quality. The operation of the Dynamic Noise Suppressor is, basically, a rather gradual one, especially in closing down the frequency range after loud passages, lest a too-evident change in background noise become audible. The Dynamic Noise Suppressor is incapable of suppressing transient noises save in a generalized manner by its relatively long-term modification of the pass band; it copes much more successfully with continuous "white" noise than with noises of a more pronouncedly transient nature. These same observations apply to such recent developments and extensions of the Scott principle as are embodied in U.S. Pat. Nos. 3,678,416 (Burwen) and 3,803,357 (Sachs). A different method of subjectively reducing surface noise is described in U.S. Pat. No. 3,275,326 (Welsh), in which two identical recordings playing simultaneously are auditioned.
Compressor-expander systems are well known in the art of sound recording and reproduction. These systems function by compressing the dynamic range in the recording process, and by expanding the dynamic range in the playback process to the extent it was compressed. The following U.S. Pat. Nos. disclose various compressor-expander systems: 3,665,345 (Dolby), 3,729,693 (Dolby), 3,732,371 (Burwen), 3,813,559 (DeBoer), 3,815,039 (Fujisawa), 3,828,280 (Dolby), 3,829,715 (VanSluys), and 3,846,719 (Dolby). The systems are only effective for reducing noise in those sound recordings which are specifically pre-encoded for such systems.
Other assorted noise and signal reduction systems are disclosed in U.S. Pat. Nos. 2,736,771 (Hanson), 2,912,571 (Jacobsen), 3,171,901 (Clemency), 3,394,235 (Schott), and 3,896,465 (Kaizu).